According to their interview request email, I believe it is between 8-30 people that they expect to pull from their alternate list after the class has been filled. There has been movement off of the alternate list already, but I don't know how much.
They continue to interview through the end of June, then they compile a ranked alternate list. At that point they actually go down the list. Until then the adcomm will choose anyone in any order from the alternate list.